Stratospheric air intrusions drive new particle formation in the upper troposphere

New atmospheric particles form when stratospheric air intrudes into the troposphere below, according to a new study, revealing a previously unrecognized mechanism for new particle formation (NPF) in the upper troposphere. The finding suggests that NPF in these regions aloft occurs frequently and over large geographic regions, representing an important source of particles in the free troposphere. Aerosol particles smaller than one micron in diameter are abundant in the troposphere, the lowest layer of Earth’s atmosphere. These particles serve as condensation sites for water vapor, contributing to cloud formation and, thus, play an important role in climate by influencing Earth’s radiative balance. NPF – the process in which atmospheric gasses, like sulfur dioxide and organic molecules, cluster into particles – is frequently observed near convective clouds formed by rising warm air. It is thought that this uplift brings chemical precursors from nearer to Earth’s surface into the free troposphere and clears out existing particles that might otherwise interfere with NPF. However, NPF in the upper troposphere cannot be fully explained by convective clouds alone. New particles found near the tropopause, in mixtures of tropospheric and stratospheric air, suggest other NPF mechanisms. Using data from global airborne measurements taken during NASA’s North Atlantic Aerosols and Marine Ecosystems Study (NAAMES) and Atmospheric Tomography (ATom) missions, Jaioshi Zhang and colleagues provide observational evidence of a previously unrecognized NPF mechanism occurring in the upper troposphere. Zhang et al. show that when stratospheric air descends into the troposphere, it mixes with more humid tropospheric air, resulting in elevated hydroxyl radical (OH) concentrations. This mixing occurs near the tropopause, where sulfur dioxide (SO₂) concentrations are high. This combination of elevated SO₂ and OH leads to enhanced sulfuric acid concentrations, which drives NPF in this atmospheric region. The authors speculate that the increased SO₂ abundances in this mixing zone may result from aircraft emissions and tropospheric SO₂. According to the findings, these stratospheric air intrusion events happen frequently and over large geographic areas, particularly in the remote marine regions of the midlatitudes, and therefore significantly contribute to the number of atmospheric particles in the free troposphere above these regions. “Defining the pathways of new particle formation is particularly important because the burden of this process on the atmosphere needs to be correctly represented in Earth system models if the preindustrial climate state is to be separated from that of the present day,” writes Hugh Coe in a related Perspective.